Electronic states and optical transitions in modulation-doped n-type ${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$As/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$As multiple quantum wells

We report the results of a detailed spectroscopic investigation of the optical properties of modulation-doped ${\mathrm{Ga}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$As/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{In}}_{1\mathrm{-}\mathrm{x}}$As multiple quantum wells. The emission processes are dominated by the radiative recombination between the free carriers and the photogenerated holes occurring at k=0. The absorption edge is blue shifted to the Fermi level of the electron plasma owing to exclusion-principle effects. A simple model for the calculation of the emission line shape reveals information on the energy parameters of the optical transitions. Finally, we provide direct evidence for absorption enhancement at energies close to the Burstein-Moss edge, disappearing around 50 K, which is ascribed to many-body interactions in the two-dimensional electron plasma.